The avoidance of air pollution has motivated industry to prevent discharge into the atmosphere and rather to capture often commercially valuable, particulate matter. Such dust collection is distinguished from simple air filtration in that collection requires periodic dust removal so as to maintain efficient cleaning and to accumulate the collected dust. Industrial dust collection equipment generally removes collected particulate matter from the filtering medium using either high pressure air pulses or jets or mechanical shaking mechanisms.
The cleaning mechanisms are better understood with reference to commonly used equipment. Industrial dust collection equipment is generally constructed in the form of a housing divided into inlet and clean air chambers. The two chambers are commonly separated by a partition having a plurality of apertures. Woven bags of suitable fabric, often on wire frames, are the usual filtering medium, and one is usually suspended beneath each aperture. When the mechanical shaking mechanism is used for cleaning, a plurality of bags are usually hung from a single movable member. Several movable members with such bags comprise a collection system. Various motors, gears, and eccentric or bellcrank members commonly make up an assembly for physically shaking all bags attached to the movable members. The shaking assembly is controlled so as to sequence the shaking of the bags attached to each movable member in a serial or other such fashion. Such physical shaking cleaning mechanisms were most common years earlier. The systems included many movable parts and in our contemporary economy would be quite expensive to manufacture and also to maintain.
In a system using high pressure reverse air pulses, high pressure air is directed through a blow pipe passing over nozzles leading to a plurality of bags. Alternatively, high pressure air is piped to individual valves in proximity with the open end of respective bags. The reverse pulses provide sufficient reverse air to overcome forwardly moving air and provide at the leading edge of the pulse sufficient pressure to slightly shock the bags thereby loosening caked dust. The dust cake falls from the bags to the bottom of the inlet chamber of the housing for removal either intermittently or continuously with, for example, a rotary airlock.
Collector systems using high pressure cleaning mechanisms require a large volume of compressed air. Often the compressor for an industrial plant cannot supply enough air for effective cleaning. Consequently, a separate purchase of an air compressor is not uncommon. Additionally, piping systems of high pressure air are expensive and difficult to construct. If a piping system becomes very long, pressure within the piping and any losses adds more expense with respect to number and size of compressors needed.
Another disadvantage with systems using high pressure air is that the dew point of air is elevated as it passes through a compressor. As the air cools after leaving the compressor, it loses its ability to retain moisture, and water condenses in the compressor's after cooler or piping system. Such moisture can create severe problems for dust collectors in that dust can cake or if ice forms, the cleaning hardware may become immediately inoperative. Often, an air dryer must be provided to remove the moisture. Such dryers, of course, add further system expense and maintenance requirements.